VTHF-300M | Pro Audio Review (October 1998)

Pro Audio Review (October 1998)
By Bascom H. King

Known more for musical instrument devices, microphone preamps and compressor limiters, the Demeter Amplification Company has a relatively new product, the VTHF-300M 300 W mono tube power amplifier; two are needed for stereo.

The output tube plate current is adjusted by front panel bias controls for each output tube by connecting a DVM between a ground tip jack and the tip jack for the output tube to be adjusted. This adjustment panel is behind a removable access plate the mounts flush with the front panel. The variable feedback adjustment is also on this test panel in the form of a small control like the bias adjustments.

In Use

I set the amps up in my listening room. The speakers used were Tannoy Churchills and Dunlavy SCIIIs. I fed the amps either through a First Sound Passive II preamp or fed the source straight into the amps and used their volume controls. Signal sources were various live tapes made on DAT and reel to reel, CDs and vinyl records. for my first listen, the amp was in the ultralinear mode with maximum feedback. I was impressed right away with the sound of the VTHF-300Ms. There was great ease, clarity and spaciousness to the sound, along with terrific dynamics. Tonal balance was honest and believable, and irritation was very low.

Bass was deep and powerful. The amps had plenty of usable power and would play material with high peak to average ratio at realistic levels without strain. Since the gains and output impedances are so close in the two modes of ultralinear and triode, comparing the sounds of the two modes is easy. It is recommended that the amps be turned off for switching modes, however. After trying the ultralinear and triode modes and the different amounts of feedback, I settled on the ultralinear mode with maximum feedback as my overall preference.

Summary

All in all, the VTHF-300Ms are good-sounding amps, I liked them very much and was most unhappy to let them go. I had no trouble during the review period with the amps. They performed without a hitch both in the lab and in my listening room.

Bascom H. King is a consultant To a number of high-end audio companies, having worked throughout his career with tube and solid-state circuitry, including designing various preamp circuits, hybrid power amps and tube regulators. He is a regular contributor to Pro Audio Review.

On The Bench

The signal circuitry of the VTHF-300M is similar to the old Williamson tube amplifiers. Input is applied to the first stage, which is a grounded cathode amplifier with its plate output direct coupled to a second stage configured as a split-load phase inverter. These two stages use the two halves of a 12AX7 twin triode tube.

Outputs of the driver stage are capacitor-coupled to the output stage. Four pairs of Svetlana 6550C output tubes are used in a push-pull parallel arrangement and are connected in ultralinear or triode mode depending on the position of the switch labeled "Pentode/Triode" on the front sub panel. Overall and adjustable negative feedback is taken from the output transformer secondary to the first stage cathode.

B+ voltage for the output stage is a healthy 600 V. The main filter capacitor is a series parallel arrangement of 12 220 uf at 400 V units for an equivalent capacitance of 660 uf at 800 V. The energy storage of this supply is about 120 Joules, an impressive amount. Output tube idling current is set at about 30 m A per tube, which results in about 28 W plate dissipation, well below maximum ratings. Before commencing with measurements I carefully adjusted the plate currents to 30 mA at sn AC live voltage of 120 V for both amplifiers. When the amps are fully warmed up, they draw 2.7 A from the AC line.

Frequency response was measured in the ultralinear mode with feedback at max and min, and triode mode with feedback at max. The response is for open circuit, 8-, and 4-ohm loading on the 8 ohm output tap. The ultralinear mode has more open-loop gain and higher output impedance without feedback whereas the triode mode has lower output impedance and lower open-loop gain. The net result is about the same gain and output impedance in both output stage modes.

Figure 3 shows 1 kHz total harmonic distortion plus noise as a function of output power and load on the 8-ohm output tap along with SMPTE IM distortion with 8-ohm loading. This data was taken in the ultralinear mode and with maximum feedback. Total harmonic distortion plus noise as a function of frequency at a number of power levels was also measured with feedback at maximum in the ultralinear mode. This same measurement in the triode mode gave similar results up to 100 W output. My measurements also showed how damping factor varies with frequency and amount of feedback in the triode mode. The results as alluded to above were similar for the ultralinear mode. This test is done by injecting a constant 1 A of current into the output of the amplifier under test and the resulting voltage being equal to the output impedance. All in all, the VTHF-300Ms measured pretty good with results that are typical of a well designed tube power amplifier.